Not all universities have academic staff in all fields. The letter means that they do not have an academic staff member available to supervise your research in your chosen field. Your supervisor(s) expertise need to either be directly or indirectly related to your research area.
If you are keen on this particular research topic, you'll need to research the available academic staff interests and ideally get in contact with them before applying for positions. When you contact them you should indicate your proposed start date as that will also determine whether they can supervise (i.e. they might be on study leave, maternity leave etc).
When I was applying in 2010, I had to contact and get approval from two potentials at each university I was applying for. Some of the first ones I contacted were unable to provide supervision but were kind enough to direct me to other staff that could be helpful. In the end, I ended up with two fantastic supervisors who work remarkably well together.